Many of monomers having reactive unsaturated bonds can produce polymers by selecting a catalyst for cleaving the unsaturated bonds and causing a chain reaction and appropriate reaction conditions. Such monomers having unsaturated bonds are typified, for example, by versatile monomers including vinyl aromatic compounds such as styrene, alkylstyrene, and alkoxystyrene. In addition, a wide variety of resins are being synthesized by polymerizing each of the vinyl aromatic compounds alone or copolymerizing the vinyl aromatic compounds with each other.
However, applications of polymers resulting from such vinyl aromatic compounds are mainly limited to fields of relatively inexpensive consumer appliances. The polymers are hardly applied to advanced technologies, which require high functionality and high thermal and mechanical characteristics, such as printed circuit boards in electrical and electronic fields. One of the reasons is that a thermal characteristic such as heat resistance or thermal stability and processability such as solvent solubility or film moldability cannot be attained simultaneously.
As for a method of solving such drawbacks in conventional vinyl aromatic-based polymers, Patent Document 1 discloses a soluble polyfunctional vinyl aromatic copolymer obtained by polymerizing a divinyl aromatic compound with a monovinyl aromatic compound at a temperature of 20 to 100° C. in the presence of a Lewis acid catalyst and an initiator having a specific structure in an organic solvent. Further, Patent Document 2 discloses a production method for a soluble polyfunctional vinyl aromatic copolymer having a molecular weight distribution controlled by subjecting monomer components containing 20 to 100 mol % of a divinyl aromatic compound to cationic polymerization at a temperature of 20 to 120° C. using a Lewis acid catalyst and an initiator having a specific structure in the presence of a quaternary ammonium salt. The soluble polyfunctional vinyl aromatic copolymer obtained by the technology disclosed in each of those two patent documents is excellent in solvent solubility and processability and can be used to provide a cured product having a high glass transition temperature and being excellent in heat resistance.
The soluble polyfunctional vinyl aromatic copolymer obtained by each of those technologies has itself a polymerizable double bond and hence is cured to give a cured product having a high glass transition temperature. It can therefore be said that the cured product or the soluble polyfunctional vinyl aromatic copolymer is a polymer excellent in heat resistance or a precursor thereof. Moreover, the soluble polyfunctional vinyl aromatic copolymer is copolymerized with any other radical polymerizable monomer to give a cured product. Moreover, the cured product is also a polymer excellent in heat resistance.
From the viewpoints of compatibility during copolymerization of the soluble polyfunctional vinyl aromatic copolymer with any other radical polymerizable monomer, and heat discoloration resistance after curing, the compatibility or solubility with a highly versatile (meth)acrylate compound is insufficient, and the thermal stability to high process temperatures is also insufficient. Thus, some kinds of (meth)acrylate compounds give opaque compositions in many cases, resulting in a difficulty of uniform copolymerization of the (meth)acrylate compound with the soluble polyfunctional vinyl aromatic copolymer. This causes a drawback of reducing the degree of freedom of designing for compounding formulations. In addition, failures such as blistering and discoloration occur through a high thermal history of around 280 to 300° C. in some cases.
Meanwhile, Patent Document 3 discloses a production method for an isobutylene-based polymer having a hydroxy group at the termination end, the method involving subjecting cation polymerizable monomers each containing isobutylene to living cationic polymerization at a low temperature in the presence of a halogen-containing organic compound having a specific structure serving as an initiator and also serving as a chain transfer agent and an Lewis acid to synthesize an isobutylene-based polymer having an isobutyryl group at the end, and further performing a Friedel-Crafts-type reaction of the isobutylene-based polymer having an isobutylene group at the end with a phenol-based compound having a specific structure in the presence of a Lewis acid. However, there has been a problem in that the isobutylene polymer having isobutylene at the end synthesized by the production method has no pendant vinyl group owing to the use of no polyfunctional vinyl aromatic compound, and hence a molded product has a low glass transition temperature and is not applicable to advanced technological fields, which require high functionality and high thermal and mechanical characteristics, such as electrical and electronic fields. Further, the terminal group introduced in the production method is a phenolic hydroxy group derived from a phenol-based compound.
Further, Patent Documents 4 to 6 each disclose a phenol aralkyl resin obtained by allowing an aromatic compound containing a phenolic hydroxy group to react with an aromatic compound containing at least two or more ethylenically unsaturated bonds.
Still further, Patent Document 7 discloses a soluble polyfunctional vinyl aromatic copolymer obtained by copolymerizing a divinyl aromatic compound with a monovinyl aromatic compound, in which the copolymer has a linear hydrocarbon group or an aromatic hydrocarbon group via an ether bond or a thioether bond as part of terminal groups. However, such end-modified soluble polyfunctional vinyl aromatic copolymer is also insufficient in compatibility, heat resistance, and the like in some cases. In addition, the end group having an ether bond disclosed in Patent Document 7 has been introduced through a reaction mechanism in which an OH group in an OH group-containing compound such as benzyl alcohol reacts with a polymer chain end.    Patent Document 1: JP 2004-123873 A    Patent Document 2: JP 2005-213443 A    Patent Document 3: JP 04-20501 A    Patent Document 4: JP 3206672 B2    Patent Document 5: JP 08-73570 A    Patent Document 6: JP 08-259665 A    Patent Document 7: JP 2007-332273 A    Patent Document 8: JP 2003-306619 A    Patent Document 9: JP 2006-240292 A    Patent Document 10: JP 2005-213443 A    Patent Document 11: JP 11-61081 A    Patent Document 12: JP 05-86136 A    Patent Document 13: JP 2002-20441 A    Patent Document 14: JP 2006-193660 A    Patent Document 15: JP 2002-509273 A
Meanwhile, plastic is used in a large amount in various industries such as the automobile industry, the household appliance industry, and the electrical and electronic industry. The reason why plastic is used in a large amount is based on its light weight, low cost, optical characteristics, and the like in addition to its processability and transparency. However, plastic is softer as compared to glass and the like and hence has drawbacks such as a large sensitivity to surface scratches. In order to solve those drawbacks, a method involving coating the surface of plastic with a hard coating agent has been employed as generally used means.
Used as the hard coating agent is a thermocurable hard coating agent such as a silicon-based coating material, an acrylic coating material, or a melamine-based coating material. Of those, in particular, a silicon-based hard coating agent has been heavily used because of its high hardness and excellent quality. This type of coating agent is mostly used for high-value-added products such as eyeglasses and lenses. However, the coating agent has a long curing time and is expensive, and thus is far from being suitable for hard coating of a film or sheet (hereinafter, referred to as a film or the like) to be subjected to continuous processing.
In recent years, a photosensitive acrylic hard coating agent has been developed and utilized. A photosensitive hard coating agent is cured immediately by irradiation with light such as ultraviolet rays to form a hard coated film to thereby provide a high processing speed and has excellent performance such as scratch resistance, leading to reduced total cost. Thus, the photosensitive hard coating agent is a primary hard coating agent at present. In particular, the photosensitive hard coating agent is suitable for continuous processing of a film or the like made of polyester or the like. There are exemplified, as materials for a film or the like made of plastic, polyester, polyacrylate, acryl, polycarbonate, vinyl chloride, triacetylcellulose, and polyether sulfone. A film or the like made of polyester, polycarbonate, and acryl is one kind of most widely used films or the like because of its various excellent features. Such film or the like made of a resin is widely used for a glass scattering preventing film, a light shielding film for automobiles, a surface film for whiteboards, a system kitchen surface antifouling film, or the like, and for a functional film or sheet for touch panels, liquid crystal displays, CRT flat televisions, and the like as an electronic material. In all of those applications, the hard coating agent is applied in order to prevent a surface from being easily scratched (Patent Documents 8 and 9).
In addition, in a display such as a CRT display and an LCD display using a film or the like coated with the hard coating agent, a problem arises in that reflection degrades the visibility of a display screen, with the result that eyes are easily tired. Thus, some of applications need a hard coat treatment having a surface reflection preventing ability. There is exemplified, as a method of preventing surface reflection, a method of preventing reflection by coating a film or sheet with a dispersion of an inorganic filler or an organic microparticle filler in a photosensitive resin to make irregularities on a surface (AG treatment), a method of preventing glare and reflection based on a difference in refractive index by providing a high refractive index layer and a low refractive index layer as a multi-layer structure in the stated order on a film or sheet (AR treatment), or an AG/AR treatment method using the above-mentioned two methods in combination.
It should be noted that Patent Document 1 and 10 each disclose a soluble polyfunctional vinyl aromatic copolymer obtained by subjecting a divinyl aromatic compound (a) and a monovinyl aromatic compound (b) to polymerization at a temperature of 20 to 100° C. in the presence of a Lewis acid catalyst and an initiator having a specific structure in an organic solvent. In addition, the patent documents disclose that the soluble polyfunctional vinyl aromatic copolymer is excellent in solvent solubility and processability, and can be used to provide a cured product excellent in heat resistance and thermal stability. However, none of the patent documents teaches applications to the hard coating agent.
In a photosensitive resin composition for hard coating used for a film substrate made of polyacryl, polyester, an MS resin, or the like used for a film or sheet provided with a hard coated layer, a polyfunctional resin is often used in order to increase a crosslinking density and improve hardness. Thus, because the photosensitive resin composition for hard coating has high hardness but is insufficient in flexibility and stiffness, cracking occurs during bending as secondary processing or failures such as floating, peeling and cracking occur during punching processing. Therefore, the improvement of secondary processability has been a critical issue. Further, the blending of the polyfunctional resin in a large amount results in large curing shrinkage and high polarity. Thus, in low-polar, poorly adhesive resins such as an MS resin, polyacryl, and polycycloolefin, in particular, there have been problems in that drawbacks such as insufficient adhesiveness and warpage of a hard coated film or sheet occur. In contrast, when the adhesiveness is emphasized, there has been a problem in that the use of a resin having low polarity and having a small number of functional group leads to insufficient improvements in hardness and scratch resistance. Therefore, a hard coating agent for a film or sheet substrate made of a poorly adhesive MS resin, polyacryl, polycycloolefin, and the like is insufficient in terms of adhesiveness, scratch resistance, and punching processability. Accordingly, there is a demand for a photosensitive resin composition for hard coating, by which those problems can be solved, and a cured product thereof, a hard coated film or sheet, and an optical device and an image display device having such hard coated films or sheets laminated.
Hitherto, in general, an acrylic resin or a cycloolefin resin excellent in transparency and light resistance or a polycarbonate resin excellent in heat resistance has been heavily used as a resin for an optical member. Meanwhile, the resin for an optical member utilized in fields of optics and electronics requires heat resistance and mechanical characteristics during mounting processes to electronic substrates and the like and high-temperature operations, and an epoxy-based resin has been frequently used as the resin. However, as a result of the growth of application fields of an optical member in recent years, there is a demand for a resin having more improved optical characteristics, heat resistance, and mechanical characteristics, having more excellent moldability, and having less characteristic change due to an environmental change than ever before. For example, in fields of optics and electronics for vehicles, various monitors and sensors are considered to be installed in response to electronic control and system modularization, and an optical member to be used in each of the monitors and sensors requires high reliability.
However, the above-mentioned optical resin material satisfies only a part of various characteristics necessary in advanced technical fields, which require the resistance to the use in such harsh environments, that is, various characteristics such as low water absorbency, heat resistance, moldability, mold transfer property, and mold release property. Thus, in the present circumstances, although the resin material has excellent degree of freedom of shape, lightweight property, and moldability as compared to a glass material, the resin material is not used as a material for an optical member in advanced technical fields, which require such strict environmental resistance characteristics, optical characteristics, and moldability, and the glass material is mainly used instead.
Hitherto, investigations have been made on a curable resin composition containing a polymerizable unsaturated monomer having a cycloalkyl group as a material solving the above-mentioned drawbacks of the resin material which has been used for an optical member in a large amount and being excellent in transparency, heat resistance, and low water absorbency (Patent Documents 11 to 13).
Patent Document 14 discloses a resin composition containing a (meth)acrylate having an aliphatic hydrocarbon group having 4 or less carbon atoms in an ester moiety as a monomer component (A), an alicyclic polyfunctional (meth)acrylate as a monomer component (B), and a polymerization initiator (D), in which the resin composition is cured with heating or light.
Patent Document 15 discloses a polymerizable organic composition containing (a) an aromatic monomer having at least two vinyl groups, (b) a polythiol monomer having at least two thiol groups, and (c) an anhydride monomer having at least one ethylenically unsaturated group, provided that a polymerizate of the polymerizable organic composition has a refractive index of at least 1.57 and an Abbe number of at least 33 for the purpose of attaining an improvement in a refractive index.